Most mass spectrometers in use today basically operate discontinuously; they deliver mass spectra at rates which nowadays are generally between one and a maximum of twenty mass spectra per second. If daughter or granddaughter ion spectra are measured, the scan rate sinks considerably. There are, as yet, no commercially available mass spectrometers which can record and deliver a hundred or more spectra per second. Time-of-flight mass spectrometers with orthogonal ion injection can operate with 5,000 to 15,000 individual spectra per second, which are digitized in transient recorders and added in real time; but, for reasons connected with the spectrum quality, dynamic range of measurement and reading speed, it is necessary to acquire and add together several hundred mass spectra before a mass spectrum can be read out of the digital memory of the transient recorder. Even today, it still takes at least five to ten milliseconds to read out the mass spectrum with its hundreds of thousands of values; if a hundred mass spectra were sampled per second the whole time would be taken up solely with the reading out. Since the trend is to higher digitization rates and thus to longer value sequences for a mass spectrum, no improvement is to be expected here.
Despite the discontinuous operation of most types of mass spectrometer (at least those with separate ion sources and mass analyzers), a mass spectrometer usually has an ion current somewhere between the ion source and mass analyzer which is more or less continuous, or, depending on the type of ion source, sometimes also pulsed. This ion stream is generally used to fill an ion storage device from which the ions are delivered to the discontinuously operating mass analyzer. If the mass spectrometer is connected to a separation unit such as a chromatograph, the ions from various substance peaks of the separation unit become mixed in this ion storage device to a greater or lesser extent, depending on the separation speed.
U.S. Pat. No. 5,811,800 discloses a storage bank for ions which can temporarily store ions of consecutive substance peaks generated from the substance stream of a separating device, such as a liquid chromatograph, in order to feed the stored ions, time-matched, to a mass spectrometric analysis each time without them mixing further with ions of another substance peak. This makes it possible, to a certain extent, to temporally decouple an optimal mass spectrometric analytical method from the separation method. It is thus possible not only to subject the ions from a chromatographic substance peak to a mass spectrometric measurement but also, if it proves useful, to acquire daughter ion spectra of selected and subsequently fragmented parent ions, or also to acquire granddaughter ion spectra of selected daughter ions in order to carry out definite identification of the substance or to elucidate the primary structure. Then the analysis of the ions of the next substance peak begins.
The storage bank disclosed in U.S. Pat. No. 5,811,800 cannot accumulate ions, however. It cannot store identical fractions of ions from consecutive separation runs in the same storage cells because the storage cells arranged in series can only be filled from the preceding storage cell, and thus do not permit a second filling with ions from the same type of fraction from a subsequent separation run.
The terms able to accumulate or accumulating shall mean that it should be possible to later add more selected ions to those collected earlier in the ion storage devices, and that other ion storage devices can also be filled in the meantime, with other ionic species, for example.
The ever increasing speed of separation methods creates a need for storage banks able to accumulate ions. It is thus to be expected that there will be separation methods on chips that carry out a complete electrophoretically assisted chromatographic separation run in only one second, but which separate only very little substance each time. Therefore the aim is to develop an accumulating fraction sampler to increase the dynamic range of measurement. The duration of the substance peaks may amount to only a few milliseconds.
Different ion species are separated even faster by their ion mobility in gas-filled drift regions. In this case, a single separation run takes only about 20 to 100 milliseconds, sometimes even less. The duration of the separated ion peaks also is in the order of only a few milliseconds or even less, especially with low-pressure drift regions.
As already explained above, there is, as yet, no mass spectrometer that can analytically follow ion peaks that are changing so rapidly, or which are so sensitive that they can manage with the small ion quantities in the peaks. For such fast separation methods it is therefore desirable to be able to collect identical ion fractions from consecutive separation runs accumulatively in a storage cell of a storage bank in order to feed the ions collected in this way to the analyzer in sufficient numbers and temporally decoupled.
U.S. Pat. No. 7,019,286 (K. Fuhrer et al.) describes a time-of-flight mass spectrometer with which extremely fast ion reaction processes can be followed. It uses a split detector that separates the long ion threads, which are injected into the pulser and which fly in a largely undisturbed formation through the flight tube region, into sections which can each be detected separately. Since the ion threads fly into the pulser in a few tens of microseconds, it is thus possible to use them to observe processes that change in time periods in the order of around ten microseconds. This time resolution is several orders of magnitude higher than the time resolution required for the separation methods used here, and so does not represent a solution to the problem.
Ion storage devices today generally take the form of RF multipole rod systems, in which the two phases of an RF voltage are alternately applied to the pole rods. A pseudopotential is created in the interior that constantly accelerates the ions towards the axis so that they execute oscillations around the potential minimum in the axis. If the rod system is charged with a collision or damping gas at a pressure of around 10−2 to 10+3 Pascal, the ion oscillations are quickly damped, depending on the pressure; the ions collect in a thermalized state in the axis of the rod system. The thermalization requires at least a hundred collisions with the molecules of the damping gas. At a pressure of 10−2 Pascal the damping takes around one millisecond; at a pressure of 10+2 Pascal the ions are damped in less than one microsecond. The ends of the rod systems are generally closed by diaphragms with DC potentials so that the ions are confined in the interior. It is also possible to close them with pseudopotentials generated by RF voltages across multi-electrode systems, in which case it is possible to store ions of both polarities without switching the voltages.
The term mass here refers to the charge-related mass or mass-to-charge ratio m/z, which is the only one of importance in mass spectrometry, and not simply to the physical mass m. The number z is the number of elementary charges, i.e. the number of excess electrons or protons which the ion possesses and which act externally as the ion charge. All mass spectrometers can measure only the mass-to-charge ratio m/z, not the physical mass m itself. The mass-to-charge ratio is the mass fraction per elementary ion charge. Analogously, the terms light and heavy ions refer to ions with low or high charge-to-mass ratios m/z respectively. The term mass spectrum relates to the mass-to-charge ratios m/z.
There is a need for a storage bank for ions.